In recent years, a liquid crystal projector and a DLP projector, which are both based on digital light processing technology, are widely used. Either a short arc type metal halide lamp or a short arc type high pressure discharge lamp could be used as a light source for image projection. FIG. 6 is an explanatory cross sectional view of the structure of a high pressure discharge lamp 10. The high pressure discharge lamp 10 has a spherical light emission section 4 formed in a central part thereof and sealing portions 5 formed at both ends of the light emission section 4. A pair of electrodes 11 are arranged inside the light emission section 4. In each sealing portion 5, part of the electrode 11, and a metallic foil 6, which is connected to a base end of the electrode 11, are buried, thereby forming an airtightly sealed structure. In such a high pressure discharge lamp 10, it is possible to suppress a spread of an arc by making the mercury vapor pressure thereof high at time of lighting. It is also possible to further increase an optical output.
However, since the light transmission of the light emission section 4 of the high pressure discharge lamp 10 falls as lighting time passes, there is a problem that an illuminance maintenance rate thereof decreases remarkably. Such illuminance maintenance rate decreases are deemed to be mainly caused by a blackening of the light emission section 4, which is caused when tungsten (a structure material that evaporates at time of lamp lighting) adheres to an inner wall of the light emission section 4. Therefore, as disclosed in a Japanese Patent Application Publication No. 2001-319617, the purity of tungsten of such electrodes 11 is increased to 99.99% or more, so that even if the lamp is lit for a long time, the light emission section 4 is less likely to easily denitrify, whereby a life span of the high pressure discharge lamp 10 may be lengthened.